Page 426 - Complete Wireless Design

Wireless Issues

Wireless Issues 425

Minimize high-impedance nodes, since these areas will add noise caused by
induced currents created by EMI propagation, as well as ground loops, which
create noise and EMI through induction. Adding extra ground planes between
signal tracks of a multilayer board will shield other tracks from electromag-
netic coupling, thus reducing cross talk, and permit increased AC coupling to
the ground reference plane. It is always wise to check all traces on a PCB
design for cross talk at the early layout phase. This can be accomplished by
employing field solver or cross talk software. Openings made for connectors,
LEDs, switches, and thermal vents within metal enclosures or ground planes
can act as slot antennas, especially if the opening is greater than one-tenth
wavelength, radiating the fundamental and/or its harmonics.
Keep the power supply separated from the digital and analog sections by
shielding and decoupling. Shield the synthesizer and the VCO to prevent
EMI from entering the oscillator circuit, causing spurs in their output. The
use of power planes, which are separate copper layers on a multilayer board
that are each attached to the hot end of the power supply, at RF frequencies
can cause them to become patch-like antennas, sending spurious signals
around the immediate PCB area. Utilizing simple traces for supplying the
DC, with adequate decoupling, is all that is recommended for dealing with
most high-frequency designs.
Attempt to physically separate high-speed digital circuits from the analog
circuits, or allow the digital circuits to function only when the analog circuitry
is not being affected adversely, with the RF traces located as far from the digi-
tal traces as possible. Modern high clock speeds mean that the propagation
velocity is longer than the clock cycle, so propagation delay becomes a large
consideration. If this is taken into account, then the digital clock pulses can
arrive properly at multiple chips and pins with equal delay.

10.3.4 Board design issues
Even if an MMIC or discrete amplifier is designed to be unconditionally sta-
ble, poor PCB layout can cause stability problems. Subband oscillations (below
the amplifier’s bandwidth), out-of-band oscillations (above the amplifier’s
bandwidth), and in-band oscillations may all occur unless:

1. The amplifier is connected directly to ground.
2. The PCB board itself must be connected at one-eighth wavelength points to
the system ground (and preferably closer).
3. Vias must be placed from the PC board’s top ground plane to the bottom
ground plane to lessen the reactance between these two planes.
4. V must be properly decoupled by using both low- and high-value capaci-
CC
tors for both high and low frequencies.
5. All amplifier stages with high gain should be shielded to prevent them from
bursting into oscillation.

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